Electron Transport Through Coupled Quantum Dots below the Kondo Temperature

Conductance G through coupled quantum dots in series is investigated below the Kondo temperature, based on the slave boson formalism of the Anderson model with the antiferromagnetic spin-spin coupling J. Electron transport is characterized by the ratio of the dot-dot tunneling coupling V_C to the level broadening in the dots Δ (dot-lead coupling). When V_C/Δ < 1, the Kondo resonance is formed in each dot and lead and G is determined by hopping between the two Kondo states. They are replaced by a spin-singlet state in the dots for low gate voltages. The gate voltage dependence of G has a sharp peak of 2e²/h in height in the crossover region between the spin-singlet and Kondo states. When V_c/Δ > 1, the Kondo levels are split below and above the Fermi level in the leads for low gate voltages. The gate voltage dependence of G has a broad peak, which is fairly robust against J. This broad peak is divided into two peaks when the energy levels are different between the dots.